The conventional process for preparing a pigment involves four steps: (a) diazotizing an aromatic monoamine or diamine, (b) coupling a substituted or unsubstituted naphthol or an acetoacetylated aromatic monoamine with the diazotized aromatic monoamine or diamine, (c) conditioning the reaction mixture to induce crystallization of the pigment and (d) recovering the crystallized pigment from the conditioned reaction mixture.
Conventionally-produced pigments are quite satisfactory for use in solvent-based inks, and it is well-known in the prior art to incorporate one or more surfactants during the conditioning step to aid in the dispersion of the pigments, to reduce the grind sensitivity of dried pigment preparations, etc., see U.S. Pat. No. 5,352,281.
Environmental pressures are causing printers to phase out the use of solvent-based inks and to substitute water-based inks in their stead. However, the color value of a pigment in a water-based ink and the gloss and density of such ink are generally lower than that obtained in a solvent-based ink containing such pigment.
The problems alluded to above are especially prevalent when the pigment is a barium- or calcium-laked pigment or a diarylide pigment. For example, efforts are known in the prior art to replace all or part of the calcium ions in a laked pigment with ions of heavier atoms. While such approach appears to reduce the instability problem, the resultant pigment is more extended which leads to greatly reduced efficiency.
It is also known to employ electrostatic and steric stabilization approaches. In the former case, a sufficiently high charge density is induced by adsorbing ionic surfactants which then prevents close approach of the pigment particles. In the latter case, adsorption of a polymer onto the surface of the pigment particle prevents agglomeration.